So I've mixed up my Dunwich cards and components with Arkham Horror. Now if I want to teach a newbie how to play, I do not fancy separating out the components (I find it time consuming to thoroughly mix the cards). I know by having expansions, I'm making the game harder. What should I do to keep the newbie 'safe' while travelling Arkham and yet I do not fancy removing every Dunwich component?

it would be fairly easy to retain mixed-in components and not use the Dunwich board or mythos cards. I do this all the time. Here's the trick:

In the Dunwich Mythos deck, there aren't any in-arkham gates which are not gate bursts.

So rather than looking for the tiny barn symbol on all the cards, just look at the gate openings, remove all Dunwich gates and all the red gates. You're done, simple as that.

What about the characters and ancient ones? Would any character be ok? Should I limit the Ancient one to Nyarlathotep/Ithaqua?

As awp said, the only things you need to remove are the Dunwich board, and the Dunwich mythos cards. Everything else can stay.

Which AO you want to play against is up to you. I'd pick one of the easier ones from the base game, like Azathoth or Nyarlathotep.

Keep the monsters mixed in. Hope for a pre-game Colour Out of Space draw .

Unfortunately for you, Dam, spawn of Azathoth, the Dunwich Horror won't be available (thank the Guardians). Neither will the hard masks if Nyarly isn't the Ancient One. After my times of teaching the game (without the expansion), I have decided Ithaqua is the weakest of the lot.

What about Arkham Encounters that gives stuff like Rail Passes? What about those condition cards? What do I do with them?

Knuckles Eki said:

That's 6 Arkham Encounter cards, and one of them is a Dunwich Encounter, so just 5 cards. If you want to dig them out, just look in the neighborhood color that matches the card back and find that encounter. (If you're taking out the Sheldon Gang Memberships too...that's 2 more cards.)

Frankly, I wouldn't worry about them. Just ignore them and draw again if any of them actually come up.

jgt7771 said:

...and that's exactly what I'd do whenever a card (of any type) from Dunwich is drawn: ignore it and draw again.

It's what I always do if I'm too lazy to sort the cards out.

I definitely would remove the Dunwich monsters, though. They tend to be more dangerous than the basic monsters. And since they don't have the Dunwich symbol, you cannot easily remove them later.

As a final comment: If you're really worried about the increased difficulty when playing with new players I definitely would recommend to sort the cards out beforehand. Telling a new player to ignore some of the cards will just confuse them. Laziness is a bad excuse if you're trying to get someone new interested in the game.